I was attempting to install a gem, capybara-webkit, and kept getting errors on OSX 10.8.2. Viewing the gem output log, it was attempting to create a file at /Users/jack/.ccache/a/1/<random letters> and getting a 'permission denied' issue. You can't sudo bundler, but sudo gem install capybara-webkit executed just fine.

My solution seemed to fix things:

sudo chown -R airbnb /Users/jack/.ccache